Based on combination of polyols and polyisocyanates as raw materials, structure of a urethane resin can be designed to meet the required characteristics, including soft to hard resins. For such reasons, the urethane resin is used in a broad range of industrial fields. Furthermore, use of the active energy ray curable resin to be cured by ultraviolet ray (UV) or electron beam (EB) is widened in recent years due to the characteristics like productivity, energy saving, and low environmental load compared to a thermocurable composition and a solvent-based resin composition. Among the flows of those technological innovations, research and development of an active energy ray curable urethane acrylate, which is a urethane resin having an unsaturated group like acrylate bound to the end, in particular, research and development of application techniques thereof, has been actively carried out.
Being an active energy ray curable resin, urethane acrylate is expected to be used widely for coating on various substrates, a hard coating agent, an adhesive, a cohesive agent, a sealing agent, an ink, or the like. However, a huge problem lies in that structure design of a compound to meet the various needs and adjustment of a composition blend to satisfy the expected properties have not progressed sufficiently. Namely, it is very difficult to maintain balance between the physical properties like toughness, elongation property, high hardness, and high adhesion property of the urethane structure and the physical properties like curing rate according to active energy ray curing, surface dryness and surface hardness after curing, scratch resistance, and curing shrinkage property. As a result, presently there is no resin that can satisfy the required high performances when the target is to achieve both the thin film and high function in a rapidly growing field, for example, adhesion of an optical film in display and touch panel field, hard coating for film including decoration field to optical field, hard coating field, and optical cohesive layer field.
For example, in Patent Literature 1, a photocurable resin composition containing, as an essential component, urethane acrylate with functionality of 6 or higher is disclosed. A cured coating film with tack-free surface, excellent hardness, scratch resistance, and chemical resistance by using this composition is disclosed. Furthermore, suggested in Patent Literature 2 is adduct type urethane acrylamide which is obtained by reacting acrylamide containing a hydroxyl group with polyisocyanate. Also suggested in Patent Literature 2 is oligomer type urethane acrylamide having polyol skeleton which is obtained by a reaction additionally including polyol. As disclosed in Patent Literature 2, by modifying the polymerizable group from an acrylate group to an acrylamide group, the curing rate of the adduct type is enhanced by two times or more. Furthermore, the curing property and stickiness on cured film surface of an oligomer type are improved. Furthermore, in Patent Literature 3, a windable hard coating film for molding that can have suppressed tack and blocking in non-cured state by using an active energy ray curable resin obtained by a reaction between acrylamide containing a hydroxyl group and an isocyanate compound is disclosed. Furthermore, in Patent Literature 4, a hard coating layer of an in-mold molded film having excellent surface hardness and bending property by using a curable resin composition containing acrylamide containing a hydroxyl group, trimethylol propane, a polyvalent isocyanate compound, and a reaction catalyst is disclosed.
However, in none of those Patent Literatures 1 to 4, mention is made regarding the curing resistant shrinkage property and bending resistance. Furthermore, nothing is described regarding the solubility at the time of use in combination with general purpose monomers and oligomers, or resins, and also regarding the transparency of a cured layer obtained therefrom. As such, the aforementioned problems are not solved in the present state.
In Patent Literatures 5 and 6, an oligomer or a polymer of urethaneacrylamide, which is obtained by reacting acrylamide containing a hydroxyl group, polyol, and isocyanate, is used. In Patent Literature 5, an optical film having excellent heat resistant stability is disclosed. In Patent Literature 6, a material for electrophotographic device by which crack prevention can be achieved due to increased strength is disclosed. In Patent Literature 5, an optical film having specific structural unit as being a urethane acrylamide polymer obtained by using specific polyol and specific isocyanate, and achieving the high heat resistance of the film are disclosed. In Patent Literature 6, by containing an acrylamide group, enhancement of fracture strength, improvement of a curing property, and a property of preventing cracks as a result of them are provided. However, in those Patent Literatures, the curing resistant shrinkage property, bending resistance, solubility, and transparency are not described. The aforementioned problem of not having the balance of the urethane acrylamide compound and the problem of not being able to respond to the high performances that are required in rapidly growing fields still remain unsolved.